Performance monitoring: User

[Fost97]

Hello everyone!
My goal is to associate a user to a transaction (as showed in the screenshot).

First thing first, thing that i haven't understood, is if the user shown in the transaction is the "user executing the action" or a user internal to Sentry.
As second question, relative to the first one, is that i have this implementation in my code (ExpressJS):

const express = require('express')
const Sentry = require('@sentry/node')

const app = express()

Sentry.init({
  dsn: 'dsn-variable',
  release: 'backend',
  environment: 'production',
  integrations: [
    // enable HTTP calls tracing
    new Sentry.Integrations.Http({ tracing: true }),
    new Sentry.Integrations.Express({ app })
  ],
  tracesSampleRate: 1.0
})

app.use(Sentry.Handlers.requestHandler())
app.use(Sentry.Handlers.tracingHandler())

// app.use(...)

app.use(Sentry.Handlers.errorHandler())

How to i set an "indipendent" user for every transaction generated by my integrations? because using Sentry.setUser(...) will set the user globally.

I hope I have explained myself, thank you in advance for your support!

[AbhiPrasad]

@Fost97 if you set your user inside a request handler or middleware, it should only attach the user to a transaction for that specific request!

[AbhiPrasad]

You can call Sentry.setUser(X) - this will attach the user to any outgoing transactions or errors!

[Fost97]

But if i set the user with Sentry.setUser() it will set the User globally or ONLY for the transaction that is executing?

[AbhiPrasad]

if you call it in the request handler/middleware, it'll set it for the transaction that is executing - otherwise it'll be global.

[Fost97]

Okay, i tried and it seems to work. But how? I mean, how can the transaction detect, inside the middleware, that i am setting the user (assigned to the Sentry instance and not to the transaction)? Wouldn't Using Sentry.setUser within the middlewaret cause concurrency errors to happen where multiple running endpoints assign different users at the global sentry level?

[AbhiPrasad]

Under the hood, the SDK uses async local storage to isolate sentry data per requests. See: https://docs.sentry.io/platforms/node/configuration/async-context/.